1. Field of the Invention
This invention relates to a semiconductor device with air-bridge interconnection and a method of producing the same.
2. Description of the Prior Art
A semiconductor device with air-bridge interconnection is known. Such air-bridge interconnection interconnects electrodes formed on neighbouring mesas formed on a substrate, and particularly interconnects lines crossing each other. That is, the air-bridge interconnection isolates an interconnection line from elements, such as transistors by holes or grooves. After devices are formed on a semiconductor substrate using ion implantation, an air-bridge is built over the groove between mesas to form the interconnection line. In the normal atmosphere, the air isolates these interconnection lines or elements therebetween.
If isolation between an interconnection metal and other portion is made by a dielectric substance or insulator, line capacitance therebetween becomes larger. The air-bridge interconnection can reduce the line capacitance because the dielectric substance or the insulator is the air.
The method of producing the air-bridge interconnection is disclosed in Gallium Arsenide-Materials, Devices, and Circuits, John Wiley & Sons (1985), Chichecter, U.K., at page 485. This method will be described with reference to FIGS. 9A to 9C. FIGS. 9A to 9C are cross-sectional views of a prior art semiconductor device for explaining the method of producing the air-bridge interconnection in order of steps of processing. In FIG. 9A, a substrate 601 on which electrodes 602 are formed, is covered with a resist 603. Then the resist 603 is cured. Portions of the resist 603 on the electrodes 602 are removed. Then, on the electrodes 602 and the cured resist 603, an interconnection metal layer 604 are deposited as shown in FIG. 9B. The interconnection metal 604 is formed in a desired shape. Then, the resist 603 formed under the interconnection 604 is removed by an organic solvent. The interconnection forms a bridge over the groove between the electrodes 602, so that the line capacitance of the interconnection metal 604 is reduced.
However, there is a problem that in the semiconductor device having mesas formed by wet etching or dry etching up to the order of several micron meters, the air-bridge interconnection cannot be used.
Moreover, there is also problem that disconnection of the interconnection metal occurs because the resist 603 does not perfectly enter the isolation grove having several micron meters depth or the resist 603 formed under the interconnection metal 604 is not removed perfectly, so that the reliability of the semiconductor device decreases because the leavings of the resist 604 absorbs the humidity.
Further, the resist 603 must be removed by dry etching because if an organic solvent is used for removing the resist 603 formed on the electrodes 602 (wet etching), necessary portions of the resist 603 will be removed. The dry etching requires fine control of etching time to prevent the mesa, that is, the electrode 602, from damage, so that processing is complicated.